The invention generally relates to protection of power system busbars from internal faults (short-circuits). More particularly, the present invention improves relay""s sensitivity to internal faults and stability under external faults by detecting saturation of Current Transformers (CTs) and applying extra security measures to prevent malfunction of the protective relay upon detecting the saturation.
Power system busbars are electrical nodes that interconnect a plurality of circuits such as transmission lines, transformers and generators. Consequently, busbars are connected to a number of energy sources that all together can produce enormous fault current in the event of a short circuit on a busbar (an internal fault for the busbar relay) or in a near vicinity of the busbar (an external fault for the busbar relay). Large magnitude of the fault current imposes demanding requirements on the speed of operation of busbar relays. At the same time, large currents associated with near external faults may saturate one or more CTs causing problems for the busbar protective relay. Busbars can be protected from internal faults by detecting faults internal to the protected busbar and initiating trip command to appropriate Circuit Breakers (CBs). The CBs subsequently disconnect the defective busbar from all the connected circuits in order to minimize further damage to the involved electrical equipment and the power system as a whole.
The busbar protection is typically accomplished using a differential protection principle. With reference to FIG. 1, the CTs are used to measure the currents in all the circuits connected to the busbar. Locations of the CTs define a xe2x80x9czonexe2x80x9d of protection for the bus. The currents are then compared and the unbalance (differential) current is produced as an algebraic sum of all the input currents. In an ideal operation of the CTs, the differential signal equals zero during normal load conditions and external faults, while it equals the fault current during internal faults; a simple threshold would enable the relay to distinguish between internal and external faults in such ideal circumstances.
As a practical matter, however, the CTs operate accurately only up to certain magnitude of their primary currents. If the primary current becomes too large and/or it contains slowly decaying dc (direct current) component(s), the CT becomes saturated and the secondary current is no longer an accurate indicator of the primary current. The errors introduced by the saturated CTs create as a spurious differential current during external faults. This may cause malfunction of the differential relay during external fault conditions.
To prevent relay malfunction due to CT saturation either the so called xe2x80x9chigh impedance bus differentialxe2x80x9d or xe2x80x9clow impedance biased (percentage) bus differentialxe2x80x9d principle is used.
A high impedance bus differential relay typically uses a special stabilizing resistor to create a high impedance path for the differential current. If one of the CTs saturates, its magnetizing branch has a much lower impedance relative to the effective input impedance of the relay and majority of the spurious differential signal flows through the saturated CTs, not through the relay. This prevents malfunction during external faults.
A low impedance biased bus differential relays use a special reference (restraining) current to detect excessive differential current and initiate tripping. The restraining signal is created to reflect the external fault current. The differential current is not compared against a fixed threshold (unbiased differential) but against the restraining signal (biased or percentage differential). The comparison uses operate/no-operate regions on the differential current - restraining current in a two-dimensional plane. Such a differential characteristic is typically shaped by a constant pick-up threshold and two or more slopes and breakpoints.
In addition, other means of stabilizing the relay during external faults are known. They include harmonic restraint or direct analysis of the waveform of the differential current, linear couplers or gapped transformers, CT saturation detection, and currents from the periods of saturation-free operation. Such solutions doe not provide adequate sensitivity, reliability, and/or protection against malfunctions.
The present invention overcomes the problems noted above, and provides other solutions, by providing, in exemplary embodiments, a method and system for a microprocessor-based bus differential protective relay that ensures fast and sensitive operation under internal fault conditions and offers improved stability during external fault and other non-internal fault conditions.
According to one embodiment, the method uses a two-slope differential characteristic to cope with small CT transformation errors without reducing relay sensitivity. The second (higher) slope is produced by a straight line crossing the origin of the differential-restraining two-dimensional plane. Thus, the slope provides a true percentage (proportional) restraint across all the restraining currents. The discontinuity between the first (lower) and second (higher) slopes is removed by a special transition (joining) line of the characteristic.
The maximum of all the input currents is used to produce the restraining signal for the differential characteristic.
The method can also include a two-stage software-based saturation detector that is capable of detecting saturation of one or more current transformers during external faults. The saturation detector triggers only on external faults and is capable of detecting saturation that happens as soon as approximately 2 milliseconds past the inception of the fault.
The method can also use a current directional principle for better stability on external faults. A set of currents having the highest magnitude is selected during a given fault and the angular relations between those currents and the sums of all the remaining currents are checked. If at least one of the currents is flowing out of the protected busbar as compared with the direction of the sum of the remaining currents, the external fault case is declared and the relay is inhibited.
The method can also use a saturation detector to switch dynamically between the 1-out-of-2 and 2-out-of-2 operating mode depending whether or not the CTs saturate. In a 2-out-of-2 operating mode, both the differential and directional elements must confirm a fault for the relay to trip. In a 1-out-of-2 mode, if saturation is not detected, the relay can trip without checking the fault direction.
The differential characteristic can be divided into two sub-regions, a lower region operating in the fixed 2-out-of-2 mode, and an upper region operating on the 2-out-of-2 basis if CT saturation is detected.